G4: two windows on one workbook without copying it; Arrange All

- The workbook's active sheet is an atomic set in place (Workbook::active_sheet /
  set_active_sheet), so a window swap or sheet switch never clones the
  Arc<Workbook> or marks it dirty. Selections already live in DocState and the
  window views. Save records the sheet of the most recently active window.
- A dialog taken out of its window while it runs goes back to that window when
  a command run from it switched windows (dialogs::show, ui.dialog.confirm).
- view.arrangeAll {arrange?: tiled|horizontal|vertical|cascade, activeWorkbook?}
  returns the visible windows, active first; the desktop moves and sizes the
  native windows over the main window's monitor. New Window and Arrange All
  buttons on the View ribbon; Arrange Windows dialog.
- Docs: ui-parity, hardware-parity, gaps #20, target-app-parity, ROADMAP.

crates/engine/src/cmd/file.rs

@@ -185,9 +185,9 @@ fn close(s: &mut Session, p: &Json) -> Result<Json> {
 fn export(s: &mut Session, p: &Json, ext: &str) -> Result<Json> {
     let path = str_param(p, "path").ok_or_else(|| bad("file.export", "missing `path`"))?.to_string();
     let d = s.doc()?;
-    let mut wb = (*d.wb).clone();
+    let wb = (*d.wb).clone();
     if p.get("sheet").is_some() {
-        wb.active_sheet = target_sheet(s, p)?;
+        wb.set_active_sheet(target_sheet(s, p)?);
     }
     let target = if path.ends_with(&format!(".{ext}")) { path.clone() } else { format!("{path}.{ext}") };
     let bytes = crate::io::save_bytes(&wb, &target)?;